The purpose of the proposed studies is to examine the relations between the redundant sensory signals and varying biomechanical arrangements that influence postural stability. Are the segmental restabilizing actions preprogrammed in a global fashion to maintain the body's center of mass over the base of support, or are they continually calculated to match local changes in sensory input? Changes in body mechanics, local sensory inputs, and/or prediction of upcoming events are considered important components of postural control during normal daily activities. Clarification of how each of these mechanisms influence the reaction to postural instability should assist clinicians in evaluating causes of falling and balance disturbances. Such clarification will improve interventions directed toward training patients to compensate for, or adapt to postural disorders. To accomplish this goal, both normal subjects and bilateral labyrinthectomized patients will be tested. Subjects will be rotated and translated in the pitch plane in a variety of postural states (e.g., sitting, standing) with the axis of rotation either at the head, the center of mass, or the ankle. Subjects will also be perturbed with head and/or trunk fixed when standing on a posture platform that can be servoed to the ankle angle. This will permit selective elimination of vestibular, neck, and ankle proprioceptive inputs in order to measure the effects of each sensory signal on the response. Subjects will be asked either to voluntarily stabilize against the perturbing force, or to distract themselves with mental arithmetic. Two stimulus waveforms will be used. A discrete, predictable ramp wave will provide information about the latency and magnitude of the relevant responses. Pseudorandom ramps will evoke more automatic characteristics of the response. Specific questions relating to the overall goal include studies that identify the mechanical parameters regulating the spatial and temporal sequence of stabilizing reactions, examine the influence of anticipation on patterns of muscle activation, and determine the linear realtions between sensory signals and functional stability.